Why does the German Army like 20mm auto cannons so much?

[davebender]

More specifically, why does the German Army like the 20mm x 138mmB / 20mm x 139mm cartridge so well? They have used it in a wide variety of weapons from the 1930s right up to the present day. Not that I have anything against this cartridge but why do they prefer it over readily available auto cannon cartridges that are more powerful?



Panzer II ausf C. Mass produced 1937 to 1940.
Historically armed with KwK30 or KwK38 auto cannon chambered for the 20mm x 138mmB cartridge. Not a bad light tank but why wasn’t it armed with the much more powerful 3cm Mk101 / Mk103 auto cannon?
Armor Penetration.
49mm @100 meters. 20mm x 138mmB cartridge. Same for KwK30 and KwK38.
75mm @ 300 meters. 30mm x 184mmB cartridge. Mk101 cannon.
70mm @ 300 meters. 30mm x 184mmB cartridge. Mk103 cannon.

RH202 auto cannon. 20mm x 139mm cartridge.
Main gun for Marder IFV, Luchs recon vehicle, various flak mounts etc.

The RH202 is ok but what happened to the 30mm MG213C revolver cannon that was operating in prototype during 1945? Britain copied this weapon as the ADEN cannon and it has been very successful for dozens of nations. The MG213C has a more powerful round, a higher rate of fire and yet the weapon weighs only 87kg. What’s not to like? Germany created a fantastic 30mm weapon yet the Heer chose to stick with a slightly improved version of their WWII era 20mm cartridge right into the 1980s.

[Shortround6]

1.Because the MK 101 didn't exist when the 20mm x 138B cartridge was adopted.
2. The 30mm guns were bigger and heavier, the MK101 was almost three times heavier than the 20mm guns.
3. Size and weight of the ammo. The MK II tank carried about 180 rounds of 20mm ammo in 18 ten round magazines. It used a one man turret ( many descriptions are wrong). while tanks are not really weight sensitive they are very volume sensitive. MK 101 could be feed with 6 round boxes but given the size of the ammo the magazines are going to be bigger and heavier and harder to change, gun breech takes up more room in the turret. Using a belt feed means you cannot change the type of ammo from AP to HE or from normal AP to an APCR or APDS round.

RH202 auto cannon. 20mm x 139mm cartridge.

In standard loading's the older round had about 47,000 joules of muzzle energy, the new round has 72,600 joules for the HE round and 84,000 joules for the AP. A bit more than a "slight" improvement. The 20mm round for the MG213C had a muzzle energy of 61,700 joules. The rate of fire for RH202 is 800-1000rpm vs the 1200-1400rpm of the WW II gun. Without a 350-700mph slipstream of cooling air for the gun barrel high rates of fire from a single barrel can be a real problem. The New gun incorporates a dual feed. Two belts go into the feed mechanism and the gunner can select which belt (type of ammo) in 1-2 seconds. This is harder to do with a revolver cannon.

[davebender]

1935.
3cm MG101 developed by Rheinmetall-Borsig. It was evolved from an existing 20mm AT weapon.
…..Note. MG101 was original weapon designation and used through 1940.

1937 to 1940.
1,113 Panzer II ausf C produced.
…..Assumption. MG101 cannon not yet production ready.
…..Point of Departure. The Panzer II turret will be designed for the 3cm MG101 cannon. The 2cm cannon will be employed as an interim weapon just as the 3.7cm cannon was employed as an interim weapon on the Panzer III. Consequently the Panzer II turret (including ammo storage) is sized to fit the larger weapon. Heer 8 wheel armored cars (Sd.Kfz 232, 233 and 234) will also use this turret.

July 1939.
MG101 cannon tested in Me-110B aircraft. 10 round magazine.
…..Assumption. MG101 cannon production ready NLT this date. Might have been made production ready sooner but that would be speculation.
Historical Luftwaffe procurement of MG101 cannon was tiny. The Heer will fund procurement of 200 weapons per month from fall 1939 onward. Hence this cannon will become a Heer program.

September 1939.
German invasion of Poland. 16mm armor on early war Panzer IIs proved vulnerable to Polish AT rifles. Consequently frontal protection of existing vehicles was improved by adding 20mm armor plates. New production Panzer II had 30mm frontal armor built in.
…..In this scenario existing Panzer II are rearmed with the 3cm MG101 cannon while at depot for the armor upgrade.

[Shortround6]

And this gets you what when the T-34s and KVs show up?

If you can stuff a MG101 in the turret you could probably stuff in the regular 37mm tank cannon.
You need a bigger turret ring anyway.

[davebender]

32 ton T34s and Sherman tanks don't become a serious problem until 1942. Until then most armored vehicles weighed less then 20 tons and were armed with a 37mm or 45mm main gun. Threats the dirt cheap Panzer II can handle if armed with something more powerful then the historical 20mm cannon.

The 10 ton Panzer II could probably be armed with a 3.7cm/45 main gun. Personally I think the 3cm MG101 auto cannon is a superior overall weapon. Multiple hits in close proximity allow you to chew through armor. You cannot do that with a single shot weapon.

[tomo pauk]

If we really want a tank for Heer, wielding the 3cm autocannon, perhaps Pz-38(t) is better choice?

[Shortround6]

The armor penetration figures given are for tungsten cored shot. Using normal AP shot penetration falls to 32mm or under. Germans had a shortage of tungsten and while they did use in the aircraft guns for tank busting (because using the normal shot would have been near useless) equipping hundreds of tanks with guns that are going to out of ammo in short while doesn't make a lot of sense.

[davebender]

Why?

- The Panzer II ausf C and Pz-38(t) are both 10 ton tanks.
- Both have rough riding leaf spring suspension.
- 25mm frontal armor on Pz-38(t) ausf A through F is inferior to 30mm frontal armor on 1940 & later Panzer II.
- Early model Pz-38(t) use inferior riveted construction rather then welding.
- I haven't seen a production cost for the Pz-38(t). But I doubt it costs less then the dirt cheap Panzer II.

Don't get me wrong, I think the Pz-38(t) was ok for such a small tank. But IMO the similiar size Panzer II would be as good or better if it had a more powerful main gun.

[Shortround6]

Actual armor penetration figures for the MK 101 were 25mm/300m/90^ for the 500gram APHE. This was improved to 32mm/300m/90^ with the later higher velocity 455gram projectile. Both penetration ratings fell of with both angle of impact and range. The Tungsten cored stuff (355 grams) could do 75mm/300m/90 degrees (or better depending on armor)but it's performance fell of worse with both range and angle of impact. Impacting at 60^ caused it's penetration to fall to between 42-52mm.

As an example of the fall off in performance of the tungsten cored rounds in these small sizes the German and Czech 37mm guns had better performance with standard ammo at ranges over 500yds than they did with the Tungsten cored rounds.

The Pz-38(t) had a least a two man turret and the Germans may have crammed in a third man (loader). in combat this beats the heck out of the one man band act going on in the MK II turret. If you want the MK II to be an effective combat tank it needs to be all new from the top of the tracks up. Not only a new turret but a new upper hull to hold the new turret.

Dirt cheap is not always an advantage. The Czech tank was noted for it's reliability and cross country performance.

[davebender]

I doubt any tank with leaf spring suspension had good cross country performance as the ride would be rough.

Germany developed torsion bar suspension for the Panzer II during 1941 but that's too late to matter for such a small tank. Under different production circumstances it might have made a good full track APC chassis.

[Shortround6]

The engine in the 38(t) was about 25% bigger in displacement which means more torque. Nobody was doing more than 8-12mph cross country no matter what suspension without throwing the crews around. Many tanks only had shock absorbers on first and last road wheels if they had them at all. Cross country ability is also hill climbing, ground loading, and a bunch of subtle things not usually found on quick fact sheets. British Churchills could climb hills better than Shermans despite much lower top speed and the later Centurian was also better in very rough terrain than the M-46-48 series. They may have moved at a walking pace but that is better than not moving at all.

A several variants on the 38(t) chassis went to 15-16 tons. The MK II didn't go beyond 11.5 tons without a suspension change. Granted they might have put heavier springs in the heavy 38(t)s but they didn't add road wheels or change the entire suspension.

[parsifal]

Germans used their light tanks, principally the PzII and Pz 38 as Assault guns later in the war. Basically rip out the turret and replace with a gun 75mm or 76.2mm. SPGs with this armament was a better re-use of a scarce tank chassis that refitting or redesigning it with an already inadequate gun.

Pz IIs were only ever intended as stopgaps. Sounds to me this upgrading scheme would be a major investment in redesign for not much return. Germans were pinning their hopes on the Pz III/IV combination, with the PzIII equipped with a 37mm gun when designed.

Re-equipping the PzII with a redesigned experimental gun in a redesigned turret in a redesigned turret ring takses the PzII from nbeing dirt cheap, into the realm of the problematic.....

[davebender]

IMO a 50% weight increase is not a variant. It's an entirely new armored vehicle chassis even if it's derived from an older vehicle. Anyway this isn't meant to be about armored vehicles per se.

Getting back to the main topic...
Early model Marder IFVs weighed 28.5 tons. By 1989 the upgraded Marder A3 weighed 35 metric tons. Similiar in size to a T-34 or Sherman tank. So why is it still armed with a 20mm cannon? The Heer could have chosen something larger such as the M242 chain gun (i.e. Bradly IFV) or 30mm Rarden cannon (i.e. Warrior IFV) and still had room for an infantry fire team.

[Shortround6]

what is the point? None of those guns will take out a tank except from the rear. APC were/are turning into light medium tanks with an infantry component. Bigger guns mean less ammo. If you want to blow up enemy armored vehicles just put a missile or two on the vehicle. At the cost of a modern IFV the cost a missile is a cheap kill and you still have the 20mm for soft vehicles and general support. Maybe the Germans like the idea of just enough gun to do the job without giving the Marder crews delusions of grandeur about tank hunting.

[parsifal]

You would have to say, however that Marders along with the US Bradleys are an over-investment for limited protection. Maybe thats what Dave is on about, though I doubt that upgrading to a 30mm cannon will make any difference to the survivability of the type.

Israeli doctrine does not support frontline use of APCs as MICVs. They have found that in their normal operating environments, such vehicles are far too vulnerable for any such role. I have my doubts about the whole MICV concept.....riding into battle in a AT rich environment in inadequately armed and protected vehicle is a sure way to get a whole bunch of Infantryman killed IMO. The Marder /Bradley options add to the conundrum by making such vehicles expensive additions to the inventory....reduces the number that can be deployed because such vehicles are too expensive.

There were problems in the Iraq deplyment of the bradleys, though I forget what those issues actually were